How to properly block AC signals with a transistor?

Thread Starter

Bserar79

Joined Oct 24, 2018
17
Good day to everyone! :)

I'm trying to build a project in witch a positive triangle wave (V high about 6V, V low about 3V) is to be switched on different loads. The problem I am seeing is that when using a MOSFET, there is a lot of current going through it at about 500KHz (the maximum frequency I need). I created a small test scenario in the simulator:

1.jpg
The simulation results are the following:
2.jpg
As we can see, there is quite some current going through the turned OFF transistor.

So, my questions are: Is this a problem related to the body diode (drain-source capacitance)? If it is so, is there a way to make such a circuit work, without current coupling through the transistor? Finally, is there some other topology/ device that may be used to completely decouple the load from ground?

Thanks in advance for all the help! :)
 

peterdeco

Joined Oct 8, 2019
391
I never encountered this problem but would try putting a series diode on the gate to only let positive through. You should also have a resistor across the source and gate to ensure the FET is off.
 

Papabravo

Joined Feb 24, 2006
16,783
M1 appears to serve no useful purpose and is behaving sort of like a capacitor. Apparently Ids is non-zero when Vgs=0, and the V2 source is putting energy in and taking it back out at a pretty good clip.
 

Alec_t

Joined Sep 17, 2013
12,200
The drain-source capacitance is probably the main current path.
Why are you switching the load supply voltage rather than the gate voltage?
 

LowQCab

Joined Nov 6, 2012
1,096
Stating what the actual problem is that You are trying solve
would probably get You some more definite answers.

It's quite common here in these Forums that people tend to get stuck on
making their "unusual-solution" to a problem work,
rather than working on figuring-out why there is a problem in the first place.

Start from the beginning, and then state what the desired outcome is.
.
.
.
 

eetech00

Joined Jun 8, 2013
2,571
Good day to everyone! :)

I'm trying to build a project in witch a positive triangle wave (V high about 6V, V low about 3V) is to be switched on different loads. The problem I am seeing is that when using a MOSFET, there is a lot of current going through it at about 500KHz (the maximum frequency I need). I created a small test scenario in the simulator:

View attachment 244695
The simulation results are the following:
View attachment 244696
As we can see, there is quite some current going through the turned OFF transistor.

So, my questions are: Is this a problem related to the body diode (drain-source capacitance)? If it is so, is there a way to make such a circuit work, without current coupling through the transistor? Finally, is there some other topology/ device that may be used to completely decouple the load from ground?

Thanks in advance for all the help! :)
I think your seeing drain-to-source leakage current.
There will be some amount of leakage even if the gate voltage is 0v. Use a low leakage type mosfet.
 

Tonyr1084

Joined Sep 24, 2015
6,258
The problem I am seeing is that when using a MOSFET, there is a lot of current going through it at about 500KHz (the maximum frequency I need).
Your graph shows approximately 160 micro amps. That's 0.00016 amps. That's not a whole lot.

Now, to someone like me who's kind-a, sort-a familiar with LTSpice, what's the voltage on V1? Is it AC or DC? What's the wave form look like? And like Alec_t said: "Why are you switching the load supply voltage rather than the gate voltage?"

Also, sometimes the sim's don't get it right. Wondering if you're seeing something that might not be there in real life. Have you built such a circuit? We need more information on what you're trying to accomplish; so shed a little clarity on the desired outcome and likely someone here can answer your question(s).
 

LowQCab

Joined Nov 6, 2012
1,096
Do You want to turn on and off a signal,
or do you want to figure out how You can do it with the wrong parts ????
A MOSFET has the equivalent of 3-Capacitors built-in.

It seems that everyone is speculating about the reasons You may be having problems,
because You have not stated the Impedance of the Signal Source,
nor the Impedance of the destination Input,
nor the amount of Current/Voltage available for Switching,
nor the part-numbers of the Source and Destination ICs, or discreet-Parts.

This appears to be a very simple problem on the surface of things,
but You are keeping everyone in the dark.
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